1. Field of the Invention
The present invention relates to a light separation element which is used in an opto-magnetic disk apparatus or the like for separating light, having polarized components angularly spaced 90 degrees from each other in the rotating direction, into three components and a light receiving optical device using the light separation element.
2. Description of the Prior Art
In an opto-magnetic disk apparatus, polarizing separation is performed to obtain a playback or reproduced output such that light returned from a disk is separated into two light components representing respective polarized components due to a Kerr rotatory angle and a light component for error signal detection from which a focus error signal and so on are detected.
FIG. 11 shows arrangement of an optical system of an opto-magnetic disk apparatus using conventional polarizing separation means.
A laser beam emitted from a semiconductor laser 1 is converted by a collimator lens 2 into a parallel beam which is reflected by a beam splitter 3 and then by a total reflection prism 4. After that, the beam is focused by an object lens 5 onto the recording surface of a disk D. The light reflected by and returned from the recording surface of the disk D passes through the beam splitter 3 and is separated by a Wollaston prism 6 into three components which are received via condensing lenses 7aand 7b by a pin photodiode 8 divided into six zones. Specifically, two light rays B1 and B2 which are separated by the Wollaston prism 6 to have different polarizing planes from each other are received by two light receiving portions 8a and 8b of the pin photodiode 8, respectively. An MO signal is detected from the difference between quantities of light received by those two light receiving portions. The remaining light ray B3 independently of the polarized conditions is received by a light receiving portion 8c divided into four zones, from which both focus and tracking error signals are obtained.
The above conventional light receiving optical device uses the Wollaston prism 6 as an element for polarizing separation. However, the Wollaston prism 6 comprises two pieces of anisotropic crystal, such as quartz, bonded to each other and, therefore, the production cost is expensive. In addition, a cylindrical lens adapted to produce astigmatism requires to be provided as the condensing lens 7b in order to obtain the focus error signal from the four-divided light receiving portion 8c of the pin photodiode 8, thus resulting in the increased number of optical components.